Ground engaging tools, such as, for example, bucket teeth, ripper tips and track shoes, used in construction machines operating in soil and rock, are subject to harsh two body wear. It is necessary for such ground engaging tools to exhibit a combination of high hardness throughout the tool in order for the tool to resist wear, high fracture toughness in order to prevent tool breakage, and sufficient temper resistance in order to prevent loss of hardness during operation at elevated temperatures.
Several steel compositions have been proposed for applications requiring the desirable combination of hardenability, toughness and temper resistance properties. The heretofore disclosed compositions include a relatively high amount of chromium, i.e., above 3% of chromium by weight. For example, a steel composition mainly intended for use as an excavating tool edge material for construction machines is described in U.S. Pat. No. 3,973,951 issued Aug. 10, 1976 to K. Satsumabayashi et al. This steel has a chromium content in the range of 3.0% to 6.0% by weight. Similarly, a wear resisting steel developed for use as a ripper tip material and having chromium in the range of 3.0% to 5.0% by weight is described in Japanese Patent 54-42812 issued Dec. 17, 1979 to Kabushiki Kaisha Komatsu Seisakusho. Another steel intended for use in mining buckets and other mineral processing operations, and having a composition including chromium in the range of 3.0% to 4.5% by weight is described in U.S. Pat. No. 4,170,479 issued Oct. 9, 1979 to G. Thomas et al.
Other heretofore known steels intended for use in applications requiring a combination of high hardenability and toughness require significant amounts of nickel. Examples of these compositions are disclosed in U.S. Pat. No. 2,791,500 issued May 7, 1957 to F. Foley et al, U.S. Pat. No. 3,165,402 issued Jan. 12, 1965 to W. Finkl et al, U.S. Pat. No. 3,379,582 issued Apr. 23, 1968 to H. Dickenson and, more recently, U.S. Pat. No. 4,765,849 issued Aug. 23, 1988 to W. Roberts, which discloses a steel composition having substantially high amounts of aluminum in the range of 0.4% to 1.0% by weight.
U.S. Pat. No. 5,131,965 issued Jul. 21, 1992 to J. McVicker and assigned to the same company as this instant invention, discloses a steel having high hardenability and toughness. However, McVicker uses higher chromium to attain high hardenability and temper resistance.
U.S. Pat. No. 5,595,614 issued Jan. 21, 1997 to J. McVicker and assigned to the same company as this instant invention, discloses a steel having high hardenability and toughness. However, McVicker uses coherent nitride, carbonitride, and carbide particles to attain fracture toughness.
It has been desirable to have a deep hardening steel having a martensitic microstructure having a predetermined and controlled spacing of .mu. size titanium nitride cuboids to obtain high fracture toughness. It has also been desirable to have a deep hardening steel that has a microstructure having a predetermined and controlled fine scale distribution of incoherent fine scale (10-400 nanometer size) carbonitrides to improve fracture toughness.
The present invention is directed to overcome one or more of the problems as set forth above.